It is known that certain types of security papers (used typically to manufacture banknotes, checks, papers for identity documents or passports, etc) are provided with several security devices, which usually include a watermark and/or a security thread.
Some kinds of forgery-resistant papers often have, in addition to the “classic” security devices mentioned above, additional security elements, constituted for example by so-called “plachettes”, fibers, filaments having different shapes, sizes, or granules that can be fluorescent, luminescent, magnetic, magnetizable etc, having various dimensions, sometimes even on the order of 10 micrometers.
These security elements are generally embedded within the sheet (at respective application regions) during its formation, by using feeder devices that are immersed in the forming box that contains the paper paste and within which the forming roller rotates.
Such feeder devices are constituted generally by so-called “spreaders”, which are provided with an internal chamber connected, by means of an intake port, to a duct for feeding the security elements, which are usually diluted in water and are contained within a containment tank.
The spreaders are further provided with a feeding opening, which faces a portion of the outer blanket of the roller, immersed in the forming box, so that the security elements that exit from the feeding opening can be deposited simultaneously with the paper paste onto the outer surface (wire) of the roller during the formation of the sheet.
In security paper manufacturing plants there is a collecting wire in order to allow the separation of the sheet from the wire of the forming roller.
The sheet of paper formed on the rotating forming roller is transferred onto the collecting wire and contains all the security elements applied also with the aid of suction means, which produce an appropriate partial vacuum inside the forming roller.
In some plants there is an additional forming roller or former, which is arranged, along the paper production line, upstream of the forming roller (main watermarking roller), which in a manner substantially similar to what has been described for the forming roller forms a first layer, termed vellum, having a very fine grammage, which will be attached to the surface of the collecting wire (before said wire is arranged at the forming roller), and the wire itself deposits it on the second layer obtained on the forming roller.
The two sheets, while wet, then form a single so-called two-ply sheet.
As mentioned earlier, spreaders for depositing the security elements must currently be arranged so that they are more or less immersed in the forming box, and this causes drawbacks.
In order to try to avoid problems during printing, such as partial separation of the security elements as a consequence of the action of the printing elements, with consequent soiling of the plates, the deposition of the security elements must be performed simultaneously with the deposition of a layer of paste on the forming roller.
For this reason, the spreader or spreaders must be properly adjusted and positioned at an appropriate depth within the forming box. However, this condition prevents monitoring (including visual monitoring) of the deposition of the security elements.
Moreover, any variation in the operating conditions within the forming box (density of the paste, partial vacuum inside the roller, etc) may determine a change in the conditions of deposition of the security elements (such as width of the band, density of the elements) that are difficult to detect in line, both due to the difficulty of accessing the deposition region and due to the fact that the security elements that are inserted often are not easily visible in transmitted light or in reflected light.